1. Field of the Invention
The present invention relates to a semiconductor device, and in particular to technology that is effective for application to a semiconductor device including a high electron mobility transistor (HEMT) element.
2. Description of the Related Art
There is technology where a first source electrode and a second source electrode are alternately disposed, and where a wide portion of the first source electrode, in which via holes are formed periodically, and a wide portion of the second source electrode, in which via holes are formed periodically, are disposed so as to be staggered, whereby the source electrode arrangement pitch is shortened and the dimension in the chip longitudinal direction is shortened (e.g., see JP-A-2000-332030).
There is also technology where, in a radio frequency-use field effect transistor where a comb-tooth-shaped drain electrode and a comb-tooth-shaped source electrode are mutually meshed together, a leading end portion of a drain finger extending as far as an inactive region in comparison to a drain ohmic electrode that is in an active region is retracted, and similarly a leading end portion of a source finger extending as far as the inactive region in comparison to a source ohmic electrode that is in an active region is retracted, whereby the parasitic capacitance between the drain and the source is reduced (e.g., see JP-A-2001-284367).
A HEMT is a high-speed field effect transistor where a two-dimensional electron gas layer formed by a compound semiconductor heterojunction is used as a current channel. In a HEMT, high mobility electrons are connected by source/drain ohmic electrodes, and a field effect transistor (FET) is caused to operate by controlling the current with a gate electrode. Also, because the frequency band of radio waves used in information systems such as mobile communication, wireless local area networks (LAN) and collision prevention radars ranges from the microwave range to the milliwave range, which is high, a HEMT including high electron mobility is suitable for use in circuits included in those information systems.
The present inventors studied technology where a HEMT was applied, as a switching element, to an antenna switch circuit, which is one radio frequency circuit included in a radio frequency (RF) module disposed in a mobile communication device such as a mobile telephone. In the process of their studies, the present inventors discovered the following problem. This problem will be described using FIGS. 44 to 50.
FIG. 44 is a plan diagram showing an example of a structure of a HEMT element for radio frequency amplification that the present inventors studied. FIG. 45 is across-sectional diagram along line A-A of FIG. 44. FIG. 46 is a plan diagram showing another example of a structure of a HEMT element that the present inventors studied. FIG. 47 is a cross-sectional diagram along line A-A of FIG. 46.
In the HEMT elements that the present inventors studied, a gate electrode 104 is disposed, so as to extend in a [−1-10] direction called an inverted mesa direction, between a source electrode 102 and drain electrode 103 on a semiconductor substrate 101 having GaAs (gallium arsenic) as a main component and including a (001) surface as a ma in surface. FIG. 44 shows a plan structure where the gate electrode 104 includes one finger that extends in the inverted mesa direction, and FIG. 46 shows a plan structure where the gate electrode 104 includes two fingers that extend in the inverted mesa direction. Also, as shown in FIG. 48, the source electrode 102, the drain electrode 103 and the gate electrode 104 of the structures shown in FIGS. 44 and 46 are plurally organized by a source wiring 105, a drain wiring 106 and a gate wiring 107 to form a single block, and plural blocks are organized in parallel to form the final HEMT element. FIG. 48 is a plan diagram of a structure where two of the structures shown in FIG. 46 are connected in parallel. FIG. 49 is a cross-sectional diagram along line A-A of FIG. 48. The reason why the gate electrodes 104 are made to extend in the inverted mesa direction is that the GaAs crystal forming the semiconductor substrate 101 does not have an inverted symmetry, so that, for example, the polarities of the charges generated by a piezoelectric field with respect to stress become opposite between the [−1-10] direction and the [1-10] direction, which directions are 90° different. Namely, the directions are 90° different between the [−1-10] direction and the [1-10] direction on the main surface of the semiconductor substrate 101, and as shown in FIG. 50, the threshold voltage and the temperature dependency of the current, which are the basics of the electrical characteristics of a HEMT, differ between the case where the gate electrode 104 extends along the [−1-10] direction and the case where the gate electrode 104 extends along the [1-10] direction. For example, with respect to the threshold voltage, changes occur in comparison to a state where stress is not working, and the directions of the changes are opposite and the amounts of the changes are the same between the [−1-10] direction and the [1-10] direction.
Also, when the gate electrodes 104 are patterned, in order to form plural gate electrodes 104 with the same dimension, the plural gate electrodes 104 are patterned so as extend in the same direction because it is easy for dimensional differences resulting from photolithography to arise due to the direction. Also, with a HEMT element for radio frequency amplification, because it is necessary for the input resistances of the gate electrodes to be low in order to obtain high gain, a technique is adopted where a number of gate electrodes 104 with which the necessary total gate width can be obtained are disposed using, as a reference, one gate electrode 104 including an optimized gate length.
The demand for miniaturization has been increasing with respect to HEMT elements used as switching elements in radio frequency modules. In the above-described HEMT formed using GaAs epitaxial crystal as a base, end portions of the conductive layer (semiconductor substrate 101) of one gate electrode 104 in the gate width direction are removed by mesa etching, so that electrical isolation is achieved. A gate pad 104A (see FIGS. 44, 46 and 48) for connection to contact holes from the upper layer gate wiring 107 is disposed for all the finger portions of the gate electrodes 104. For this reason, the problem that the miniaturization of the HEMT element is inhibited remains because a region for achieving the isolation and a region for disposing the gate pad 104A must be secured.